It is known in the art to utilize various vacuum powered accessories in automotive vehicles in which vacuum for operating such accessories is provided by the engine, a separate vacuum pump or a combination of both. For example, U.S. patent application Ser. No. 121,671 filed Feb. 15, 1980 and assigned to the assignee of the present invention shows a vacuum supply system that utilizes engine intake vacuum supplemented by an auxiliary vacuum pump. An electric motor, or other drive means, is actuated to operate the pump whenever the vacuum drops below a predetermined desired value. A pressure switch responsive to vacuum in the vacuum system is utilized to control operation of the vacuum pump drive means to provide the desired vacuum level.
While such systems may provide satisfactory operation, it is thought possible that the large variations in altitude at which automotive vehicles may be operated may warrent provision of an altitude compensating control. Thus it is desired to provide a control system that will efficiently deliver a high level of vacuum for accessory operation at sea level with altitude compensation that reduces the level of vacuum provided when operating at higher altitude so as to avoid excessive and inefficient operation of the vacuum pump.